fix(polys): canonicalise gcd over ZZ_I

[oscarbenjamin]

References to other Issues or PRs

Fixes #21342

Brief description of what is fixed or changed

The gcd computed over ZZ_I is now canonicalised wrt units so e.g.:

In [1]: gcd(-sqrt(-2), sqrt(-2))
Out[1]: √2

Previously this would give √2⋅ⅈ.

Other comments

Release Notes

• polys
  • gcd over the Gaussian integers (ZZ_I) is now canonicalised with respect to multiplication by I.

[sympy-bot]

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• polys
  • gcd over the Gaussian integers (ZZ_I) is now canonicalised with respect to multiplication by I. (#21353 by @oscarbenjamin)

This will be added to https://github.com/sympy/sympy/wiki/Release-Notes-for-1.9.

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#### References to other Issues or PRs
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Fixes #21342

#### Brief description of what is fixed or changed

The gcd computed over `ZZ_I` is now canonicalised wrt units so e.g.:
```python
In [1]: gcd(-sqrt(-2), sqrt(-2))
Out[1]: √2
```
Previously this would give `√2⋅ⅈ`. 

#### Other comments


#### Release Notes

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* solvers
  * Added a new solver for logarithmic equations.

* functions
  * Fixed a bug with log of integers.

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<!-- BEGIN RELEASE NOTES -->
* polys
    * gcd over the Gaussian integers (`ZZ_I`) is now canonicalised with respect to multiplication by `I`.
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[jksuom]

Looks good. Thanks!